Technical Field
The present disclosure relates generally to airfoils and particularly to active airfoils used to convey sheets of fibrous material through a draw between production areas.
Related Art
In the manufacture a continuous web of tissue paper or light-weight non-woven fibrous material, a space, commonly known as a draw generally separates the production line's drying area from the production line's winding area. In the case of paper manufacturing, the drying area may have a Yankee cylinder dryer, and the winding area may have one or more spools around which the tissue is wound into rolls. The rolls may be stored in inventory or moved for further processing. The draw is generally long enough to separate the winder from the drying area of the production machine, while allowing equipment to perform intermediate operations on the web as the web travels from the drying area and winding area.
These intermediate operations may include by way of example: calendaring (e.g. passing two or more webs through adjacently disposed rollers to produce webs of uniform thickness), caliper control (e.g. the measurement and adjustment of web unit weight and moisture), quality control (e.g. the real time scanning of web to identify holes and inconsistent fiber distribution), slitting (e.g. cutting the width of web exiting the dryer into multiple narrower widths), and re-pulping that portion of the web which is not being wound, such as the initial web output at production start-up or at a web break.
The web exiting the dryer section of the production machine is generally quite fragile and encounters destabilizing problems as the web moves through some of these intermediate operations. The web generally entrains a layer of air as the web moves through the draw. As the entrained air encounters the equipment that comprises the intermediate operations, the entrained air may become turbulent and create web fluttering. Fluttering can tear the web and generate dust, thereby diminishing the quality of web. While some of the intermediate operations may contribute to stabilizing the web; others can have a net destabilizing effect on the web's position and steadiness.
To account for sections of web instability along the draw, operators have tried to control the web as the web passes from the machine dryer section to the winder. These control devices included bowed pipes or rolls, straight pipes or rolls, and large flat plates or other similar devices. The nature of tissue is such that tissue has a surface being comprised to a multitude of pulp fibers radiating outwardly. As these fibers contact with stationary rigid devices such as rolls or pipes, the rigid devices tend to break these fibers, which results in the production of an extremely fine paper dust. This paper dust presents both a fire hazard situation, as well as a health hazard for the operators through ingestion into the lungs.
Previously, a common method for changing the web path through the tissue manufacturing process involved a rigid pipe. Whether bowed or straight a rigid pipe is generally simple to manufacture and install. However, the pipe method has several inherent problems. The web is generally in firm contact with the pipe, thus requiring additional tension to be applied to the web. Secondly, paper is abrasive, and gradually wears away at the pipe, encouraging periodic replacement. Thirdly, a web has a general tendency to remain attached to the curved surface of the pipe, thus requiring additional tension to break the web loose. Typically, dust particles will collect near the breakaway point, forming an extension of the pipe which eventually breaks off, falling onto the web and either contaminating the web or breaking the web.
Large flat plates were another previously popular web stabilizer and web transport system. Since this large flat plate generally occupies the majority of the draw between the dryer cylinder and the next machine element, the large flat plate is generally moved at time of start-up or web break to provide an unobstructed path for the web traverse to the re-pulper system broke pit. A mechanically driven member generally facilitates plate movement, which adds to the total system complexity. The large flat plate's long machine direction length is such that the web can alternately collapse against the surface of the plate, then pick up from the plate and subsequently collapse again, resulting in the generation of dust due to physical contact, which in turn adds to the total web tension. Additionally, to provide sufficient structural rigidity, the plate is generally made with some finite thickness to accommodate the inclusion of internal structural re-enforcement. As a result of this thickness, the entry and exit ends are shaped (generally rounded) to facilitate smooth entry and exit. The behavior of these curved ends is similar to that of the rigid pipe design, except that the tendency for web attachment to the adjacent surface is typically more aggressive because the radius employed is greater than that of the typical rigid pipe.
To address these problems, operators have generally replaced rigid bars and large flat plates with air foils. Air foils generally create lift by exploiting the Bernoulli Principal. In aeronautics, the airfoil is commonly the wing or propeller itself, both of which generally create a majority of lift at the leading edge of the airfoil. In the manufacture of fiber webs, an “airfoil” generally refers to an apparatus that spans the width of the web. These airfoils generally have a slot in the bottom of the airfoil that direct air parallel to both the bottom of the airfoil and the direction of the web movement. Because total pressure of a system remains constant, the high dynamic pressure of the air stream (i.e. the pressure of the air stream flowing horizontally) decreases the static (i.e. atmospheric) pressure vertically between the bottom of the airfoil and the web. As a result, the web, and the air under the web is generally drawn to this low pressure area caused by the horizontal airflow parallel to the web. In this manner, airfoils generally provide web support in the draw while reducing web contact.
Although airfoils address many of the problems of the rigid pipe and large flat plate, airflow instabilities near the web can induce web flutter and subsequent web contact with mechanical parts of the dryer, resulting in a coating disturbance or web damage. Web flutter can manifest in many forms, ranging from a violent flapping of the web to a high frequency drumming. Such flapping may be particularly prominent at the web edges. Increasing production speed demands are increasing instances of flutter. Accordingly, there is a long felt to have an airfoil configured to stabilize webs at increasingly higher speeds.